An equalizer system in general compensates frequency dependent losses that a signal experiences when passing through a transmission channel. Transmission channels include, but are not limited to, a wire, a pair of wires, an optical fibre, the reading and writing channels of a storage device like a hard-disc or optical disc, a wireless connection such as a point-to-point or diffuse infra-red or radio connection. A pair of wires includes a twisted pair, a twinax coax or a differential transmission line on a printed circuit board.
The compensation level of an equalizer system in general can be self-adaptive, fixed or programmable e.g. by a voltage or via a set of switches. A self-adaptive equalizer system continuously estimates the matching compensation level. It typically includes an adaptable filter, a control loop and an output reconstruction unit.
EP-1392001 describes how to organise a control loop in an equalizer system such that self-adaptation is achieved, independently from the transmit amplitude and the transmitted bit pattern. A feed-back control signal is generated from the equalised output of an equalizer filter. Depending on whether the output signal has been under- or over-compensated, the feed-back control signal increases or decreases, such that after a reasonable time the feed-back control signal converges to a value where matched compensation is reached. The control loop is formed by a first means for measuring a short-term-amplitude signal of the output signal, a second means for measuring a long-term-amplitude signal of the output signal and a comparator means for comparing the short-term-amplitude signal and the long-term-amplitude signal, and for determining the evolution of the feed-back control signal.
US-2002/0034221 discloses a communications receiver that has multiple stages each having a transfer function 1+Ki[fi(jω)], wherein the Ki vary with a sequential gain control methodology. This document thus teaches to compensate by making a sum per stage of the unity input signal linearly added to a function that has higher frequency gain. This known method makes multiple tuning signals in circuitry using many comparators and is relatively complex. It is not suited for low voltage operation nor for implementation on a small chip area using small transistors that have large input offset mismatches.
EP-05447018.2 describes a multistage equalizer filter for compensating a received distorted signal having an amplitude for frequency dependent signal modifications introduced by a transmission channel. The filter comprises at least one amplifying compensation stage having a gain and a saturation level, the gain being monotonically rising for at least a last decade in frequency below an upper data frequency of the received signal, and a gain control means for controlling the gain of the amplifying compensation stage such that the amplitude of the received signal amplified in the at least one amplifying compensation stage remains below the saturation level of the amplifying compensation stage. The gain may be regulated by separately regulating low frequency gain, mid-frequency gain and higher frequency gain for each of the at least one amplifying compensation stages.